IQ Data from ADC

On 30.9.15 10:53, b2508 wrote:
>> On 9/30/2015 3:30 AM, b2508 wrote:
>>>> On Tue, 29 Sep 2015 09:47:06 -0500, b2508 wrote:
>>>>
>>>>> Can I somehow obtain IQ Data by using two ADCs on same input real
>>> signal
>>>>> and are there some limitations to this?
>>>>>
>>>>> I see Texas Instrument ADCs have two channels named I and Q but I do
>>> not
>>>>> understand if I have two inputs (for two analog signals representing
> I
>>>>> and Q) or they form I and Q from real signal.
>>>>>
>>>>> Thank you very much in advance.
>>>>
>>>>
>>>> Does this help?
>>>> http://dspguru.com/dsp/tricks/complex-downconverters-at-fs-over-4
>>>>
>>>> Allan
>>>
>>> I am not sure if this is what I need.
>>>
>>> I receive IF data on one analog channel (real data) and then this goes
>>> through ADC. I get real data at the output and this goes to DDC and I
> get
>>> IQ Data.
>>> My question is, can I get IQ data from ADC and then instead of feeding
>>> real data to DDC, feed IQ data to it?
>>>
>>> I am not sure what is the benefit of this and what is the reason but
> it
>> is
>>> a requirement.
>>
>> Such a strange requirement.
>>
>> The DDC is generating I/Q data by digitally multiplying (mixing) your
>> real data stream with a complex carrier (quadrature).  You can do the
>> same mixing in the analog domain and sample the two I/Q analog signals
>> with the ADC chip you have chosen.  Is that what you are looking for?
>>
>> Seems a lot easier to do in the digital domain using your DDC chip.  Why
>
>> do you need to feed the DDC chip I/Q data?
>>
>> --
>>
>> Rick
>
> Thank you :) It is a relief that I am not the only one thinking it is
> strange requirement. I do not know the reason behind it.
>
> I also understood at the beginning that requirement is having 2 separate
> inputs, one for I and one for Q both obtained through analog down
> conversion from RF to IF but later on , I realized it is one real signal
> and now I am just confused. If it is real signal why is it not enough to
> obtain IQ data after DDC to baseband.


You will receive I and Q signals if the input is already result of
a complex frequency conversion (RF to IF). You have to use them
both if you want to suppress the images at the RF.

For details, Google for 'Image suppression mixer'.

-- 

-TV

>On 9/30/2015 2:18 PM, rigby@crd.ge.com wrote:
>> On Wednesday, September 30, 2015 at 10:38:06 AM UTC-4, b2508 wrote:
>>
>>> I am actually interested in why Texas Instruments calls their ADC
>channels
>>> I and Q?
>>
>> Sigh... Then why did you ask this question in your first post?
>
>Sighing, but no answer.  What was the point of your post?
>
>-- 
>
>Rick

Seems unbelievable to me that you didn't understand that that is
additional question.


---------------------------------------
Posted through http://www.DSPRelated.com

To all, thank you.

I understand that channels are named I and Q because they can be used as
inputs for complex analog downconversion outputs and in that case name
makes sense.
I also know what interleaved sampling is, the thing is TI explains it by
saying that at the output you get IQIQIQ... In case they named their
channels ch1 and ch2 they would say output of interleaved sampling is
ch1ch2ch1ch2... 
Instead, they say IQIQIQ... and this can be very confusing for people not
experienced in this area as they may thing that by using interleaved
sampling they can obtain i and q data and I think this is not the case.
What you get is just your real input signal sampled with higher rate.
This above is my opinion but I wanted to verify if maybe I am wrong and if
maybe somehow in a way which I don't understand, this could actually be IQ
data.
If not, I wanted to verify is there a way to obtain IQ data from ADC by
somehow.
I hope now you understand my issues. 



---------------------------------------
Posted through http://www.DSPRelated.com

>On 30.9.15 10:53, b2508 wrote:
>>> On 9/30/2015 3:30 AM, b2508 wrote:

>
>
>You will receive I and Q signals if the input is already result of
>a complex frequency conversion (RF to IF). You have to use them
>both if you want to suppress the images at the RF.
>
>For details, Google for 'Image suppression mixer'.
>
>-- 
>
>-TV

Thank you , I actually receive real signal and not I and Q.
---------------------------------------
Posted through http://www.DSPRelated.com

On Wednesday, September 30, 2015 at 3:47:12 AM UTC+13, b2508 wrote:
> Can I somehow obtain IQ Data by using two ADCs on same input real signal
> and are there some limitations to this?
> 
> I see Texas Instrument ADCs have two channels named I and Q but I do not
> understand if I have two inputs (for two analog signals representing I and
> Q) or they form I and Q from real signal.
> 
> Thank you very much in advance.
> ---------------------------------------
> Posted through http://www.DSPRelated.com

take real signal and then multiply by cos and sign in the processor or FPGA